The present invention relates to electric current conducting mechanisms for controlling a magnetic field in a high magnetic field device. More particularly, the present invention relates to current conducting and switching mechanisms for conducting axial and toroidal currents for controlling a magnetic field in a high magnetic field device. Even more particularly, the present invention relates to current conducting and switching mechanisms for conducting axial and toroidal currents for creating and controlling a toroidal magnetic field in a toroidal region, and a time-varying axial or poloidal magnetic field, which in turn produces a toroidal electric field, in a high magnetic field device, such as, for example, a tokamak fusion reactor (TFR).
The main elements of a heretofore known tokamak fusion reactor (TFR) 20 are shown in FIGS. 1, 2, 3 and 4. The TFR 20 includes a vacuum vessel 21 in the shape of a torus. A major axis 22 of a cylindrical coordinate system 19 (see FIG. 2) is a straight line centered in a hollow center 25 (or bore) of the toroidal vacuum vessel 21 (FIG. 1), i.e., inside the "doughnut hole", and a minor axis 24 of such coordinate system is a circle lying in a plane normal to the major axis 22 and at a center of a core of the toroidal vacuum vessel 21. The major axis 22 extends in a z-direction and the minor axis 24 is a distance R.sub.o from the major axis 22. Points along the minor axis 24 are defined by an angle .phi.. Walls of the torodial vacuum vessel 21 are defined with respect to the minor axis 24 by another radius r and an angle .theta.. A centerpost region 23, for example, in a TFR is defined as a cylindrical volume centered on the major axis 22 and extending radially to an innermost cylinder of the vacuum vessel 21.
The present invention relates to a novel and nonobvious approach for conducting and controlling currents in this centerpost region 23 or in a similar region.
Referring to FIG. 3, large magnetic field coils 26 (FIG. 1), commonly referred to as toroidal field (TF) coils 26 or B-coils 26, envelope the minor axis 24 and the toroidal vacuum vessel 21, with part of a current I.sub.z carried by the B-coils 26 being in the centerpost region 23. This B-coil 26 when energized with the current I.sub.z, produces a strong toroidal magnetic field B.sub..phi. that is oriented parallel to the minor axis 24 encircling the major axis 22. The strong toroidal magnetic field B.sub..phi. is used in, for example, a TFR to contain high-temperature deuterium-tritium plasma to promote nuclear fusion of the deuterium-tritium mixture.
Referring to FIG. 4, a toroidal plasma current I.sub.p that flows through the high-temperature plasma in a TFR generally flows in a toroidal direction (i.e., parallel to the minor axis 24) and is needed to improve plasma confinement and provide initial heating of the high-temperature plasma. This plasma current I.sub.p is conventionally driven by a transformer system.
In order to produce the plasma current I.sub.p in the transformer system, a time-varying current I.sub..phi. flows through an electric field coil 28, or E-coil 28, also commonly referred as an ohmic heating coil (or OH coil). The E-coil 28 serves as a primary winding of the transformer system and serves to induce the plasma current I.sub.p in the plasma, which is in-effect the transformer's secondary winding. This plasma current I.sub.p is induced by a toroidal electric field E.sub..phi. in the plasma generated in response to the time-varying current I.sub..phi.. The E-coil 28 is positioned principally within the centerpost region 23 and inside the bore of the vacuum vessel 21.
The transformer system thus provides an inductive voltage for generating and driving the plasma current I.sub.p. The plasma current I.sub.p in turn provides resistive (or ohmic) heating of the plasma. Other systems for additional heating of the plasma in a TFR are known in the art.
The plasma current I.sub.p generates a relatively constant poloidal magnetic field B.sub..theta., and, when combined with the toroidal magnetic field B.sub..phi., provides a spiral field line geometry located on closed toroidal surfaces within the plasma (the surfaces of constant magnetic flux are closed about the minor axis 24).
Referring to FIG. 5, the B-coil 26 (also referred to as the TF coil, or toroidal magnetic field coil) and E-coil 30 (also referred to as the OH coil, or ohmic heating coil), in accordance with heretofore known approaches, are separate coils partially occupying the centerpost region 23 (FIG. 1) inside the bore of the toroidal vacuum vessel 21 (FIG. 1).
A combination of the toroidal magnetic field B.sub..phi. with the poloidal magnetic field B.sub..theta. provide spiral (or helix-like) magnetic flux lines that generally lie on closed nested magnetic surfaces in the toroidal vacuum vessel 21 (FIG. 1).
Ions and electrons within the plasma rotate about the toroidal magnetic field B.sub..phi. and flow generally along the minor axis 24 (FIG. 2). As a result, the ions and electrons follow a spiraling path around the toroidal vacuum vessel 21 (FIG. 21) confining the particles away from the wall of the toroidal vessel 21 (FIG. 1).
The magnetic fields, i.e., the toroidal magnetic field B.sub..phi. and the poloidal magnetic field B.sub..theta., within the TFR provide an inward force that substantially overcomes outward pressure of the plasma and significantly confines the plasma's flow to within the toroidal vacuum vessel 21 (FIG. 1). Additionally in heretofore known approaches, plasma positioning is accomplished using poloidal field shaping coils 30 (FIG. 1).
Referring to FIG. 6, with the constant current I.sub.z flowing in the B-coils 26 (FIG. 1), the resulting toroidal magnetic field B.sub..phi. interacts with the constant current I.sub.z to produce an approximately constant force F.sub.B in the centerpost region 23 (FIG. 1). The constant force F.sub.B is directed radially toward the major axis 22 of the TFR. Another approximately radial force is directed away from the major axis 22 of the toroidal vacuum vessel 21 and varies over time. This time-varying force F.sub.E is produced by an interaction between the time-varying current I.sub..phi. flowing through the E-coil 30 and an axial component of the poloidal magnetic field B.sub..theta. produced by the E-coil 30.
Previous efforts have attempted to configure the E-coil 30 and the B-coil 26 such that the constant force F.sub.B and the time-varying force F.sub.E react at an interface 27 between the E-coil 30 and the B-coil 26 and effectively cancel, for certain magnitudes of the constant current I.sub.z and the time-varying current I.sub..phi..
The size of the TFR is critically dependent on a radial extent of the centerpost region 23 (FIG. 1). The size of the centerpost region 23 (FIG. 1) determines a capacity of the centerpost region 23 (FIG. 1) to carry the constant current I.sub.z and time varying current I.sub..phi.. The strength of the material in this region and its ability to easily conduct current ultimately limit the strength of the fields that can be produced in the TFR and thus the ability of the TFR to confine the high-temperature plasma.
Accordingly, there exists a need for improvements in current conducting and switching mechanisms for controlling a magnetic field in a high magnetic field device, such as a TFR, that, for a given centerpost region size, allow greater plasma containment forces to be generated by current flowing through the centerpost region than has been heretofore achievable using the above-described approaches.